The present invention relates to implantable, partially biodegradable polymer-containing molded articles for the administration of active substances to plants.
In contrast with human and veterinary medicine it is rather unknown to administer active substances to plants by means of implantable devices. The active substance formulations used in general practice are either sprayed on the plant or brought into the soil near the roots. These conventional application forms have the disadvantage that great amounts of the active substance is last during application; this result in a considerable environmental impact (contamination of the air, soil and waters) require larger amounts of active substances.
This disadvantageous active substance loss is of particular weight when plant protection measures are to be taken on urban regions (trees in avenues, municipal park grounds), or when they have to be repeated for several times during one vegetation period, for example, in agricultural plants with high degree of pest infestation.
In order to minimize the risks of environmental pollution during plant treatment and to achieve a prolonged duration of action of the active substances, various alternative active substance release systems have been developed. For example, those described in the patent specifications of EP 0 254 196 and DE 39 22 366. These relate to devices for the transcuticular or transperidermal application of systemic active substances to plants. These are sustained-release depot preparations in the form of flat-shaped, patch-like, adherent and pressure-sensitive adhesive systems which are suitably applied on a chosen site of the plant shoot axis. In addition, there are the active substance administration forms according to DE-GM 17 60 060 and U.S. Pat. No. 4,766,695. These are simple tree rings comprising insect repellents or insecticides giving external protection to the plant. The above-mentioned publications do not clearly indicate that these are systems for a systemic administration of active substances; however, because of their construction and composition, it is obvious that they can have this function.
The above-mentioned active substance release systems are applied externally onto the plant surface. For this reason, they have the disadvantage that their functional capability which is based on perfect adhesion is influenced by environmental factors. In addition, these devices have to be removed after depletion of the systems. Moreover, this type of application does not present a satisfactory take-up of active substances since absorption is impeded by barriers in the form of terminal tissue which is difficult to permeate.
The direct administration of active substances into the conductive system of the plant by means of an injection--as described in the patent documents U.S. Pat. No. 4,078,087; U.S. Pat. No. 4,103,456; CA 1,089,645, and U.S. Pat. No. 3,576,276--can overcome the above drawbacks; however it involves other deficiencies. Since the active substance release is effected very rapidly and directly into the conductive system, there is the potential risk of excessive active substance concentrations involving plant damages. Another disadvantage is the fact that several repeated treatments are necessary to ensure the required active substance concentration at the site of action over a longer period. These drawbacks can be avoided by using release devices positioned in the interior of the plant and ensuring continuous and long-term supply of active substances at the same time.
Administering active substances to plants by means of implantable application systems has been mentioned in the relevant literature (patent document AU 8431497 and JP 58039602); however, they have not attained commercial status so far.
The publication AU 8431497 relates to an implantable device in the form of a porous ceramic body positioned in a mechanically provided hole in the trunk and connected with an external active substance reservoir by means of capillaries. The active substance release which is effected in two steps, i.e. transport from the reservoir and passage through the porous body, is carried out by utilizing the pressure in the conductive vessels generated by transpiration and the capillary forces of the porous implant. For this reason, the main disadvantage of such an application system is the fact that the active substance release to the plant exclusively depends on the water balance of the plant, and this may result in an excessive active substance concentration if transpiration is intense. If this device is used, the active substance can scarcely be dosed with sufficient accuracy. Moreover, the application of this system is very difficult since the implant body has to be removed after termination of the treatment.
The implantable release system described in JP 58039602 can overcome these disadvantages. It relates to implantable active substance-containing molded articles of different forms (tablets, rods, discs, etc.) consisting of a mixture of highly water-absorbing polymers, such as starch-acrylamide-copolymers, starch-acrylonitrile-copolymers, and hydrophilic polymers, such as ethylene-vinyl acetate-copolymer. These systems are used to release biologically active substances to trees; amongst other things, to combat diseases requiring long-term treatments, for example in case of ceratocystis spp. (in Pinus silvestris). They are inserted into the cavity (hole) previously bored into the trunk, and remain there for a longer period of time. Swelling caused by excessive water absorption results in a considerable increase in volume, and this results in the fact that the total space available for the implant is filled and the previously formed aperture sealed up. Although the publication gives no direct indication as to a biodegradability of these implants, their chemical composition suggests that they can at least partially be degraded in the plant's organism. For this reason they must be regarded as partially biodegradable within the meaning of this publication, and they have the advantage that they possibly need not be removed from the plants.
However, these systems involve a lot of problems mainly resulting from their chemical constitution. Owing to the fact that they exclusively consist of hydrophilic polymers, they are only partially suitable for incorporating highly lipophilic substances. In particular if a relatively high active substance loading capacity is required, these implant systems are rather unsuitable.
An additional disadvantage is the fact that a hydrophilic polymer matrix, after incorporation into plant tissues with a rather high cell hydration state, releases the active substance relatively fast. This indeed creates high active substance concentrations, but the system is exhausted after a short time. In the practice of administering active substances to plants, for example, in the treatment of periodically appearing diseases with a high risk of infection or pest attack, there are many cases where an effective concentration is to be maintained over a longer period of time. For these applications (for example, scurf attack in apple trees in areas with heavy rainfall, or pest attacks destroying the sprouts in banana plants) polymers are required as carriers, which impart depot properties to the system.
Also, the problem of active substance losses could not be solved in a satisfactory manner in these systems. As mentioned above, producing an intimate contact between the implant body and the boundary surfaces of the plant cavity greatly depends on the water absorption and therefore on the hydration state of the tissue cells at the site of application. As is generally known, the osmotic conditions of plant cells are subject to severe variations according to the demands on the water balance. Extremely low or extremely high water potentials can cause great volume changes of the implant and therefore change the seal tightness of the aperture in the trunk.
Finally, the relatively poor thermoplastic processibility and insufficient mechanical properties are to be mentioned as further drawbacks of these systems; these also result from the hydrophilic character of the skeletal polymers.